1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus, especially useful for an open-type MRI apparatus that has an open magnet gantry.
2. Description of the Related Art
Earlier open-type MRI systems include a vertical field type MRI system (MRI system with open access to patient image volume), featuring open access to the patient image volume, as disclosed for example in U.S. Pat. No. 4,829,252.
In an MRI system featuring good open access, such as the above-noted vertical field type of open MRI system or a tubular type MRI system of the past with a short axis and large diameter, the patient couch is moved front and back and to the left and right at a uniform height with respect to the patient transport axis when acquiring images. In this case, in the open MRI system, there are not good gradient magnetic field linearity and static magnetic field uniformity in comparison with the earlier tubular type MRI system. However, because there is good magnetic field uniformity at the center of the gradient magnetic field and static magnetic field, if images of the region of treatment of the patient are acquired at the center position of the magnetic field, high-quality images of the region of treatment are obtained. In this case, there is needed a complicated process in which a mark from a positioning projector is set on a specific point of the patient or T/R coil before moving the patient into the gantry as a preparation of setting the patient on the center of the magnetic field. This is to inform the MRI main unit of distance information between the mark and the center of the magnetic field in a front/rear direction and a right/left direction with respect to the patient transport axis.
When operating the above-noted type of open MRI as an interventional apparatus, for example, the region (location) of treatment is often not large, but limited to a specific area. In the case, even if the patient couch is moved within a horizontal plane with respect to the patient transport axis at a uniform height, it was either extremely difficult or impossible to position the region of treatment at the center of the gradient magnetic field and static magnetic field in three dimensions.
For this reason, the images obtained from such MRI systems in the past exhibited a great deal of image distortion, image non-uniformities, and fat artifacts, making it difficult to use the images in treatment.